1. In a large bowl, combine ground beef, breadcrumbs, grated Parmesan, egg, chopped onion, minced garlic, Worcestershire sauce, salt, and black pepper. Mix until well combined.
2. Form the mixture into 6 oval patties.
3. In a large skillet over medium-high heat, brown each patty for about 2-3 minutes on each side. This helps to seal in the juices and gives the steak a nice color.
4. Transfer the browned patties to the bottom of your slow cooker.
5. In a separate bowl, whisk together the beef broth and cream of mushroom soup until smooth. Add the sliced mushrooms and pour the mixture over the patties in the slow cooker.
6. Cover and cook on low for 5-6 hours or on high for 3-4 hours, until the patties are cooked through and tender.
7. About 15 minutes before serving, mix the cornstarch and water in a small bowl to make a slurry. Stir the slurry into the gravy in the slow cooker to thicken it.
8. Serve the Salisbury steaks with plenty of mushroom gravy spooned over the top.
